My technician found the microcontroller responsible for charging. Sadly, it is the main chip, CSR8670. While it is still possible that other elements in the circuit are the cause of the problem, replacement of the CSR seems improbable. In it's datasheet it states that the chip has 16MB of flash memory on board. While the chips themselves cost around 10 to 15$, the programmer isn't even available. I don't know wether datasheet is DMCA protected so I won't post it here. The only feasible solution in case the main chip has kicked the bucket is sadly to replace the whole board if you can even find it, or just buy a new pair of headphones